Metal sheet for acoustic purposes and method of making same.



P. A. WALLER. V METAL SHEET FOR ACOUSTIC PURPOSES AND METHOD OF MAKINGSAME. APPLICATION FILED OCT. 26. I912- L Qifi fi'. Patented Nov. 7,1916.

PER ARON WALLER, 0F STOCKHOLIVI, SWEDEN, ASSIGNOR TO CARL RICHARDWALLER, OF TRENTON, NEW JERSEY.

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METAL SHEET FOR ACOUSTIC PURPOSES AND METHOD OF MAKING SAME.

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Specification of Letters Patent.

Patented Nov. "X, 1916.

Application filed October 26, 1912. Serial No. 727,972.

inafter described, the clamping of such plate, while heated, in suchmanner that the plate is of necessity stretched during the ensuingcooling, and then the cooling of the plate and consequent stretchingthereof beyond the elastic limit of the metal.

My invention further comprises the heating of steel or other metal oralloy to a temperature above the critical point of transformation ofsuch metal or alloy, and the cooling of such metal while the metal isheld against contraction. And my invention comprises as well thesounding boards for pianos, and the membranes or diaphragms of telephonemicrophones, and the like, resulting from such methods of treatment,said sounding boards and membranes, diaphragms, etc, having superioracoustic properties and being made of steel or other metal which hasbeen stretched to beyond its elastic limit 6. g. by the methodindicated.

The object of my invention is to improve the acoustic properties ofsheet steel and other metals designed to be used for the sounding boardsof pianos and other musical instruments, or for the diaphragms or membranes of telephone microphones, and for other purposes.

In the accompanying drawings I illustrate more or less diagrammaticallytwo forms of such apparatus which may be used in the stretching of steelplates in the carrying out of my process.

In said drawings: Figure 1 shows a transverse vertical section of onesuch form of apparatus, the view also showing a plate to be stretched inposition in the apparatus. Fig. 2 shows a detail fragmentary side viewof the two frames shown in Fig. 1, illustrat ing the use of wedges forholding the frames together. Fig. 3 shows a transverse vertical sectionof an alternative form of apparatus, this view also showing a plate tobe stretched imposition in the apparatus. Fig. 4 is a detail fragmentarysectional view of one of the blocks 7 and 8 of Fig. 3, illustrating on agreatly enlarged scale the roughening of the surface of such block.

Sheet metal which is to be used to amplify sound, as, for example, sheetmetal whlch is to be used as the sounding board of a piano, or as, adiaphragm or membrane of a telephone microphone, must be homogeneous,must have an absolutely fiat sur face, and must have high elasticity.Homogeneous metal is required, in order that sound waves may beamplified evenly, and in order that the sound may be clear, and that thetone may possess fullness and beauty. An absolute flat surface isnecessary in order that the vibrations may be uniform, and in order thatno additional means may be required to make the surface flat and true. Ahigh elastic limitof the metal is necessary in order that this elasticlimit may not be exceeded when the plate is put under the stress towhich it is necessarily subject in use. Sheetsteel possessing thesequalities cannot be produced by ordinary rolling or forging processes,or by any other mechanical means known to me.

I have found that if a piece of sheet steel be heated, to a temperaturesuch that, if the metal when so heated be held against contraction, themetal during cooling is stretched beyond its elastic limit; and if themetal so heated be secured against con traction and allowed to cool, sostretching it beyond its elastic limit, this steel sheet, when removedfrom the stretching apparatus, and when cold, retains the shape and theabsolute flat surface acquired in such stretching apparatus, and themetal has been rendered homogeneous and has had its elastic limitraised. By this process of stretching, any internal stresses in thesheet are equalized, and therefore the metal is brought to a uniformcondition, such that its vibrations due to sounds are uniform (whereasthe vibrations in a sheet of steel not so treated are apt to be veryirregular) and such a sheet of steel may then be used as a soundingboard of a piano, or as the diaphragm or membrane of a telephonemicrophone, and when so used will produce true, full sounds, clear intone and of a timbre superior to that produced by the aid of wooden andother sounding boards.

Referring to Fig. 1, showing one form of apparatus which may be employedin the stretching of the metal, numerals 1 and 2 designate two frames,frame 2 being so shaped that, when pressed against frame 1, the sheet ofmetal 3 between these frames is pinched at the edges and so is heldagainst contraction. These frames 1 and 2 may be special stretchingframes, or may together constitute the sounding board frame of a pianoor other musical instrument. Various means may be employed for holdingthe two frames together. I have indicated for the purpose bolts 4:,secured to the under frame 1 and passing through apertures in the upperframe 2, and provided above frame 2, with openings 5 adapted to receivewedges 6, (Fig. 2), which wedges, when driven into place, clamp the twoframes together so tightly that contraction of the metal 3 is preeluded.

In the alternative apparatus illustrated in Fig. 3, 7 designates ananvil block having a flat upper surface, and 8 designates an upper blockhaving a fiat lower surface. Block 8 may have such weight that when itis pressing the sheet metal 3 against the top of anvil block 7,contraction of the sheet 3 is precluded by the mere weight of block 8and by the surface friction of the sheet 3 against the surfaces of theanvil block 7 and upper block 8; or, the surfacesof these two blocks 7and 8 may be slightly roughened. In practice the roughening is veryslight and may be only that produced by ordinary tools (a planer toolfor example) in machining the surfaces of the two blocks. Such roughnessis too slight to appear in the drawings without exaggeration, andaccordingly in Fig. 4, an enlarged and exaggerated section isillustrated, but such slight roughness is nevertheless very effective inprecluding contraction of the sheet metal held by the said blocks.

The rate of contraction of steel in cooling between definite temperaturelimits is well known, and therefore it is easy to calculate thetemperature to which a sheet of steel must be heated in order that,after such steel has been so heated and is clamped in either theapparatus shown in Fig. 1 or in the apparatus shown in Fig. 3, it may bestretched beyond its elastic limit (the elastic limit of steels ofdifferent composition being Well known, or, if not known, being easy todetermine by well known methods) during the resulting cooling. Incarrying out my process therefore, I take a sheet of steel 3, ofsuitable dimensions, and heat it to a temperature previously determinedas such that, when the sheet cools, while held against contraction, itwill be stretched beyond the elastic limit; and I then place such sheetof steel in either the apparatus'shown in Fig. 1, or the apparatus shownin Fig. 3, and immediately clamp the sheet firmly and .permit it tocool.

It will be seen that by the apparatus of Figs. 1 and '2 the metal willbe stretched longitudinally, while by the apparatus of Fig. 3 it will bestretched both longitudibut to the contrary contemplate the treatment ofsuch steels by my process.

As a special feature of my process, though not in all cases a necessaryfeature, I include the heating of the steel and other metals and alloys,not only to a. point such that the metal will be stretched beyond theelastic limit when cooled while held against contraction, but to a pointbeyond the critical point of transformation of the metal. It is wellknown that steel and other metals and alloys when heated beyond acertain temperature (this temperature varying with different grades ofsteel and other metals,

but being well known or easily determined by well known methods, foreach particular grade) undergoes a change of crystalline structure; andthat when such steel or other metal, after being so heated, beyond thecritical point of transformation, is cooled, it has acquired and retainspermanently a new crystalline structure, characterized, usually, notonly by greater hardness, but also by greater elasticity. I have foundthat this heat treatment of metal to be used for sounding boards, etc.,viz :the heating of the metal to beyond its critical point oftransformation, before the metal is placed in the stretching apparatus,and then the cooling of the metal in the stretching apparatus while suchmetal is held against contraction, is particularly advantageous formetal to be used as sounding boards, microphone diaphragms, etc.

The apparatus shown in Fig. 3 has been found to be particularly suitablefor use when treating steel which has been heated above the criticalpoint of transformation; for the heated steel is cooled rapidly by itsintimate contact with the two cool metal bodies 7 and 8, the mass ofwhich is so great, relative to that of the steel sheet 3, that heat isabsorbed from such steel sheet very much as heat would be absorbed ifthe sheet were dipped in water or other hardening liquid.

By this means the steel plate is hardened'or tempered, and its elasticlimit greatly raised.

As a particular illustration of the temperatures to which steel may beheated to advantage, when of from .35 to .40 percent. carbon, fortreatment according to my invention, I will state that where heating beyond the critical point of transformation is not desired, heating tofrom 350.to 400 centigrade has proved effective; and when heating beyondthe critical point of transformation is desired, heating to from 800 to900 centigrade will give good results. But it is to be understood thatthese temperatures are not constant for all grades of steel but may bevaried according to the nature of the metal treated.

What I claim is:

1. The herein described method of treating sheet steel to be used forsounding boards and the like, which comprises heating the steel sheet toa temperature such that, when cooled while held against contraction, itis stretched beyond its elastic limit, and then holding along its edgesthe steel sheet so heated and permitting it to cool, and therebystretching the steel beyond its elastic limit.

2. The herein described method of treating sheet metal to be used forsounding boards and the like, which comprises heating the metal sheet toa temperature above its critical point of transformation, and then-holding along its edges the metal sheet agalinst contraction andpermitting it to coo 3. The herein describedmethod of treating metal tobe used for sounding boards and the like, which comprises heating themetal to a temperature above its critical point of transformation, andthen clamping the metal between the surfaces of metal bodies having highheat absorption capacity, and permitting theheated metal to cool whileso held.

4. The process of treating flat sheets of metal to remove unevennessestherefrom which comprises stretching said sheets longitudinally tobeyond the elastic limit of the metal while in a heated condition.

5. The process of treating fiat sheets of metal to remove unevennessestherefrom which comprises stretching said sheets both i laterally andlongitudinally to beyond the elastic limit of the metal While in aheated condition.

6. The process of treating flat sheets of metal to remove unevennessestherefrom which comprises stretching said sheets longitudinally tobeyond the elastic limit of the metal by cooling said metal sheets froma heated condition without longitudinal con-- traction.

7. The process of treating fiat sheets of metal to remove unevennessestherefrom which comprises stretching said sheets both laterally andlongitudinally to beyond the elastic limit of the metal by cooling saidmetal sheets from a heated condition without lateral and longitudinalcontraction.

8. As a new article of manufacture a metal sheet for acoustic purposesof metal stretched to beyond its elastic limit.

9. As a new article of manufacture a sounding board of steel stretchedtobeyond its elastic limit.

10. As a new article of manufacture a .fiat metal sheetfor acousticpurposes of metal stretched to beyond its elastic limit, said sheetbeing homogeneous and of high elasticity, and having the characteristicsof metal stretched to beyond its elastic limit by cooling withoutcontraction.

11. As a new article of manufacture a sounding board of steel stretchedto beyond its elastic limit, said sounding board being homogeneous andhaving flat surfaces and high elasticity, and having the characteristicsof metal stretched to beyond its elastic limit by cooling withoutcontraction.

12. As a new article of manufacture a metal sheet for acoustic purposesof metal stretched longitudinally to beyond its elastic limit.

13. As a new article of manufacture a metal sheet for acoustic purposesof metal stretched longitudinally and laterally to beyond its elasticlimit.

14. As a new article of manufacture a sounding board of steel stretchedlongitudinally -to beyond its elastic limit.

15. As a new article of manufacture a sounding board of steel stretchedlongitudinally and laterally to beyond its elastic limit. v

16. As a new article of manufacture a metal sheet for acoustic purposeshaving a thickness of less than 1 mm. and made up of metal stretched tobeyond its elastic limit.

17. As a new article of manufacture a sounding board having a thicknessof about .6 mm. to 1. mm. and made up of steel stretched to beyond itselastic limit.

18. As a new article of manufacture a metal sheet for acoustic purposeshaving a thickness of less than 1 mm. and made up of metal stretchedlongitudinally and laterally beyond its elastic limit.

19. As a new article of manufacture a sounding board having a thicknessof about .6 mm. to 1. mm. and made up of steel stretched longitudinallyand laterally to beyond its elastic limit.

20. As a new article of manufacture a sounding board having a thicknessof about .6 mm. to 1. mm. and made up of steel stretched to beyond itselastic limit, said sounding board being homogeneous and having flatsurfaces and high elasticity, and

having the characteristics of metal stretched to beyond its elasticlimit by cooling Without contraction.

21. The process of producing metal sound- 5 ing boards for pianos havingsuperior acoustic properties which comprises clamping such metalsounding board, While in a highly heated condition, in the soundingboard frame of such'piano, and thereafter 10 stretching the metal ofsuch sounding board to beyond its elastic limit by cooling Withoutcontraction.

In testimony whereof I have signed this specification in the presence oftwo subscribing Witnesses.

PER ARON WALLER.

Witnesses:

' GRETA PRI FR. N. BLoM ms'r.

